1. Field of the Invention
The present invention relates to a laminated ceramic electronic component and a method of producing the same. Particularly, the present invention relates to the structure of an external terminal electrode of a laminated ceramic electronic component such as a laminated ceramic substrate and a method of forming the same.
2. Description of the Related Art
In many cases, electronic components such as chip antennas, delay lines, high frequency composite switch modules, and reception devices are made using laminated ceramic electronic components. The laminated ceramic electronic components are mounted on appropriate mounting substrates. Accordingly, the electronic components include external terminal electrodes which are connected to the mounting substrates.
FIG. 9 is a cross-sectional view of a portion of a prior art laminated ceramic electronic component 1 which is relevant to the present invention, in which an external terminal electrode 2 is provided.
A laminated ceramic electronic component 1 includes a main member 4 composed of a plurality of laminated ceramic layers 3, an internal conductor film 5 which defines an internal circuit element provided inside of the main member 4, and a via-hole conductor 6. The external terminal electrode 2 is provided on the first major surface 7 of the main member 4 which extends in the same direction as the laminated ceramic layers 3, and is electrically connected to a mounting substrate not shown in FIG. 9.
Ordinarily, the external terminal electrode 2 is formed by baking electro-conductive paste.
In some cases, such an external terminal electrode 2 is formed by applying conductive paste on the main surface 7 of the main member 4 after sintering, and baking the conductive paste. From the standpoint of high efficiency and low cost of processing, advantageously, the baking of the conductive paste to form the external terminal electrode 2 is performed at the same time that the ceramic is fired. Therefore, preferably, the conductive paste which forms the external terminal electrode 2 is applied on a ceramic green sheet for the ceramic layer 3 or is applied on the main surface 7 of the main member 4 before firing, i.e., in the green state, and is fired at the same time that the green main member 4 is fired to obtained the sintered main member 4.
In some cases, plating with nickel and gold or plating with nickel and tin is performed.
When the laminated ceramic electronic component 1 is used in a high frequency band, especially, in a frequency band of several hundred MHz to several GHz, to reduce the insertion loss of a high frequency signal, the external terminal electrode 2, as well as internal circuit elements such as the internal conductor film 5 and the viahole conductor 6, preferably has a low resistance, is tight, and has outstanding surface smoothness. In addition, the edge angle θ of the external terminal electrode 2 must be relatively large.
The low resistance of the external terminal electrode 2 may be achieved by using a low resistant material, such as silver and copper, as a conductive component of conductive paste which forms the external terminal electrode 2. Silver and copper have a relatively low melting point. Thus, when baking the external terminal electrode 2 and firing the laminated ceramic layers 3 at the same time as described above, a low temperature sintering ceramic material, which may be sintered at a temperature of about 1000° C. or less, is used as a ceramic material for forming the laminated ceramic layers 3.
Moreover, the tightness and the outstanding surface smoothness of the external terminal electrode 2 is easily achieved by optimization of the composition of conductive paste of the external terminal electrode 2 and the baking conditions of the conductive paste.
However, it is very difficult to increase the edge angle θ of the external terminal electrode 2 to the required edge angle. This will be described below.
First, the small edge angle θ of the external terminal electrode 2 is caused as follows.
When conductive paste for forming the external terminal electrode 2 is applied by screen printing, the conductive paste adheres to the peripheral edge of a patterning portion of the screen plate through which the conductive paste is passed. Thus, the thickness of a conductive paste film of the external terminal electrode 2 is decreased in the peripheral portion.
Moreover, the conductive paste film is broken by pressing. As a result, usually, the edge angle θ of the external terminal electrode 2 is in the range of 10 to 25 degrees.
The size A of a thinning top portion of the external terminal electrode 2, which defines the edge angle θ, is typically in the range of about 30 μm, depending upon the thickness, the area and the shape of the external terminal electrode 2.
To increase the edge angle θ, the following methods have been proposed: (1) the thickness of the conductive paste film is increased; (2) a setting resin is added to the conductive paste to prevent the conductive paste film from being crushed; and (3) the thickness of the edge of the formed external terminal electrode 2 is increased by etching of metal foil, formation of metal foil by photolithography or active plating, or etching using photosensitive paste, such that the edge angle θ is increased.
However, when the method (1) is used, the difference between the firing-shrinkage behaviors of the conductive paste of the external terminal electrode 2 and the ceramic of the ceramic layers 3 is increased.
Therefore, cracks form in the sintered main member 4, voids (cavities) form beneath the external terminal electrode 2, and the electrical insulating property of the laminated ceramic layers 3 deteriorates due to invasion of a plating liquid. Thus, the Q value of the main member 4 is reduced, such that the high frequency characteristic is deteriorated. Moreover, the main member 4 is often deformed or distorted, such that the co-planarity is reduced. This deteriorates the reliability of the connection between the laminated ceramic electronic component 1 and an appropriate mounting substrate on which the component 1 is mounted, and also the reliability of the connection between the main member 4 and mounting components, such as ICs, which are mounted on the main member 4.
In the case in which the method (2) is used, the conductive paste is supplied onto the screen plate, and then, the paste may be cured component-by-component or time-dependently. Thus, the printing property of the paste is deteriorated.
Moreover, it is difficult to smoothly remove the setting resin in the firing process. This causes generation of voids or delamination.
Moreover, to dry ordinary conductive paste, it is sufficient to heat the paste at about 100° C. for about 2 minutes. On the other hand, for the paste having the setting resin added thereto, the setting resin must also be set. Thus, the drying conditions must be more severe. For example, drying is performed at a temperature of about 150° C. for about 5 minutes. Accordingly, for the conductive paste applied to a ceramic green sheet, a plasticizer, provided in the ceramic green sheet, is often removed in the drying process. Therefore, the ceramic green sheet becomes brittle. Thus, the green sheet may be broken or cracked when handled. Moreover, the ceramic green sheet, when dried, shrinks to a much greater degree. Thus, when a plurality of the ceramic green sheets are laminated, positional shifting is caused.
When the method (3) is used, e.g., processes of coating and exposing a photoresist, peeling the photo-resist, etching, and rinsing with water, the cost increases due to the additional processes required. Thus, the method is unsuitable for practical applications.
As seen in the above-description, no method of the related art for decreasing the edge angle θ of the external terminal electrode 2 is suitable.
When the laminated ceramic electronic component 1 is a multilayer ceramic substrate, for example, the size must be reduced and the wiring density must be increased, and hence, different types of circuits are provided therein, and the functions of the substrate are combined. Accordingly, it is necessary to increase the number of the external terminal electrodes 2. Therefore, the area of each external terminal electrode 1 must be reduced. The thickness of conductive paste applied to form an external terminal electrode 2 with such a small area is further reduced. As a result, the edge angle θ is further reduced.
More specifically, the following problems are caused when the edge angle θ of the external terminal electrode 2 is small.
FIG. 10 is a schematic cross-sectional view of the external terminal electrode 2 which is connected to the viahole conductor 6 in the laminated ceramic electronic component 1.
Referring to FIG. 10, when a small current flows from the viahole conductor 6 into the external terminal electrode 2, a substantial surface-skin effect is caused. Thus, as shown in arrow 8, the current flows in the vicinity of the surface of the external terminal electrode 2. Accordingly, as the edge angle θ of the external terminal electrode 2 is reduced, the loss increases. Table 1 shows the determination results of the relationship between the edge angle θ and the loss.
Edge angle 0 (degree)Loss171.00250.91320.89900.75Half-circular shape0.74
Table 1 shows the loss occurring when a high frequency signal is passed through the external terminal electrode 2. The loss is expressed by a relative value obtained when the angle θ of 17 degrees is taken as 1.00.
As seen in the Table 1, it is desirable that the edge angle θ is increased for reduction of the loss.
FIG. 11 is a schematic cross-sectional view of two adjacent external terminal electrodes 2 connected to two viahole conductors 6 in the laminated ceramic electronic component 1, respectively.
Referring to FIG. 11, while the sizes of the laminated ceramic electronic components 1 defining the multi-layer ceramic substrates are reduced, due to the reduction in size, the increasing densities, and the composite-structures of such components, the number of required the external terminal electrodes 2 is increased. Simultaneously, the gap 9 between the adjacent external terminal electrode 2 is reduced to be, e.g., in the range of about 0.1 mm to about 0.4 mm. In this case, if the edge angles θ of the external terminal electrodes 2 are small, electric fields are concentrated on the tops of the edges, such that current tends to flow between the two adjacent external terminal electrodes 2, which greatly reduces the withstand voltage. For example, when the laminated ceramic electronic component 1 is used in transmission systems of communication devices and automotive components, where large currents flow, the above-described problems are more severe.
FIG. 12 is a schematic view showing the state of the laminated ceramic electronic component 1 of FIG. 9 mounted on the mounting substrate 10.
In FIG. 12, the main member 4 of the laminated ceramic electronic component 1 is schematically shown, and the mounting substrate 10 is also schematically shown. A mounting component 11 is mounted on the main member 4. A metal cover 12 is fixed to the main member 4 so as to cover the mounting component 11.
When the laminated ceramic electronic component 1 is provided for high frequency use, for enhancement of electrical properties, it is important to secure the grounding when the laminated ceramic electronic component 1 is mounted. Thus, in the laminated ceramic electronic component 1, a grounding conductor (not shown) is arranged in the main member 4 in the vicinity to the mounting substrate 10. In addition, a grounding conductor 13 is arranged in the mounting substrate 10 in the vicinity to the surface thereof, such that the potential of the grounding conductor 13 is about the same as that of the grounding conductor provided on the main member 4 side.
In the mounting structure shown in FIG. 12, the external terminal electrode 2 functions as a microstrip line. The loss caused by the microstrip line is influenced by the edge angle θ in the edge of the microstrip line, i.e., in the edge of the external terminal electrode 2 surrounded by a broken line circle in FIG. 12. As the edge angle θ decreases, the loss increases.
Referring to FIG. 9 again, the edge angle θ of the external terminal electrode 2 is small, and therefore, the thickness of the edge is reduced. For this reason, if a low temperature sintering ceramic material including a glass component is used for the laminated ceramic layers 3, the glass component is exposed at the edge, which deteriorates the plating property of the material. Accordingly, it is difficult to properly deposit a plating film made of tin or other suitable material on the external terminal electrode 2. As a result, the soldering property is reduced, and the reliability of the connection to the mounting substrate is deteriorated. Moreover, if copper, which tends to oxidize, is used as a conductive component of the external terminal electrode 2, and the plating property is inferior, the copper is exposed and is easily oxidized. This reduces the reliability of the connection.
As described above, a small edge angle θ of the external terminal electrode 2 causes various problems. However, it is difficult to increase edge angle θ of the external terminal electrode 2 according to the known method.